In the past, molten glass has been prepared by charging into a glass melter a formulation containing all of the necessary ingredients to produce the desired end glass composition. By the prior art methods, molten glass compositions are necessarily limited to compositions containing only those constituents which can survive the environment of the glass melter. Such limitations may be eliminated by the multi-step method of glass making taught in co-owned U.S. patent application Ser. No. 680,841, filed May 5, 1976, now abandoned whereby the constituents of the desired molten glass compositions are classified into two or more constituent groups based upon their individual melting or reactive characteristics. The constituent groups are separately prepared as molten masses, one of which is chosen as a base or host glass into which the other groups are sequentially mixed and reacted to form the desired molten glass composition. It is preferable to intermix and react the additive constituent groups with the molten glass as it flows through the forehearth.
In practicing the multi-step glass making process there may be large differences between the host glass viscosity and the viscosity of the molten additives. Thus, intermixing and reacting these molten masses presents a particular problem to be overcome. The prior art teaches two basic mixing techniques for homogenizing molten glass. The molten mixture may be flowed downward through the apparatus as in U.S. Pat. Nos. 2,520,577; 2,569,459 and 2,716,023 or through a horizontal mixing region within the forehearth as in U.S. Pat. Nos. 3,057,175; 2,467,809; 3,723,084 and 3,328,150. Of these two prior art approaches the former is most efficient; however, it is necessary that the glass melter be at a suitable elevation above that of the forming positions to provide the necessary vertical drop. The latter technique, as taught in the prior art, is not suitable for homogenizing a molten additive into a host glass where the additive viscosity is significantly lower than that of the host glass and more particularly where the less viscous additive is highly volatile. A low density volatile additive will tend to surface within the forehearth and volatilize, compounding the mixing operation and creating pollution consequences.